1. Field of the Invention
The present disclosure relates to acoustic logging of formations surrounding a borehole. More particularly the present disclosure is drawn to a multi-mode acoustic source to be used with logging-while-drilling systems or wireline tools.
2. Description of the Related Art
The generation and recording of acoustic waves through a subterranean formation is a key measurement employed in wellbore logging. Sound waves are generally classified into two types: longitudinal and transverse. A longitudinal, or compression, wave is one in which the medium oscillates in the same direction as the wave propagates. A transverse, or shear, wave is one in which the medium oscillates perpendicular to the direction of wave propagation. Both types of waves and in particular their velocities are of interest in oilfield applications. Acoustic waves propagate underground at velocities that vary depending on different geological formations. For example, the compression wave travels at about 4000 meters per second (13,123 feet per second) through sandstone and about 5000 meters per second (16,404 feet per second) through limestone. A log of sound speed or velocity as a function of depth is key to geophysical inversion. Additionally the acoustic velocity depends on rock properties such as porosity, stress state, rock strength, etc. and so its measurement has important applications in geomechanics and petrophysics.
Being able to take measurements as a well is being drilled, which is called logging-while-drilling, is advantageous since it enables reduced drilling time thereby reducing rig costs. Logging-while-drilling also enables the driller to accurately correct the direction of the drill using collected and processed data sent uphole via telemetry.
Acoustic measurements in a borehole provide information related to the velocity of an acoustic wave propagating in the formation as well as other features such as alterations, stress estimation, etc. In order to make these measurements, there is a need to develop a hardware device, called a logging tool, to be deployed in any deep hole, such as an oil well or CO2 well, that is capable of generating and recording acoustic waves propagating in the borehole. A sonic logging tool typically includes a transmitter source and an array of receivers separated by a known distance. Acoustic energy is radiated from the transmitter source into the borehole medium where it excites multiple waves propagating along the borehole to the receiver array where they are recorded as waveforms. Waves propagating in the borehole environment can be divided into two types; dispersive and non-dispersive. Acoustic dispersion refers to the phenomenon that waveform slowness (reciprocal of velocity) changes with frequency. Acoustic waves whose slowness does not change with frequency are said to be non-dispersive. Examples include P&S compressional and shear headwaves. Waveforms generated from borehole modes like LeakyP, Dipole flexural and Stoneley whose slownesses change with frequency are dispersive (Sinha and Zeroug, 1997). Analysis of the dispersion characteristic of these borehole modes is the basis for many new sonic answer products.
Acoustic modes are categorized by their azimuthal order as monopole (zeroth mode), dipole (first), quadrupole (second) and so on and are generated based on the pulsing mode of a source. A monopole source shrinks and expands, thus imitating heart beats: it pushes back and forth the fluid it is in contact with. This is the most basic mode and higher order modes can be synthesized from a collection of such monopole sources. A dipole mode is generated by a source oscillating on the horizontal axis: it acts much more like a spoon stirring fluid with a single degree of freedom. A quadrupole source expands on two opposite sides while shrinking at the same time on the two perpendicular sides: this makes it resemble a ball pinched between the thumb and forefingers, and would then switch its shape to that of an American or rugby football. As each of these modes provide different information about the formation, it is useful for a sonic tool to be able to generate waves pulsing according to each of several modes, which is then called a multi-mode source. It is current practice for each mode to be generated by a different hardware source.
One requirement of a multi-mode source is to generate sufficient energy to be delivered and coupled into desired modes with an adequate signal to noise ratio (SNR) at the receivers for useful measurements. The signal to noise ratio is particularly important in high noise environments such as exist in logging-while-drilling scenarios or when the formation being excited by the acoustic wave is very attenuative (slow formation, for example). Hence there is a need for a powerful and efficient multi-mode acoustic source that remedies the weaknesses of the sources currently used.